A printed wiring board, particularly a laminated printed wiring board, is constituted by laminating a plurality of insulating substrates, each carrying a conductive layer, with each other. The conductive layer provided between the insulation substrates is electrically connected to other optional conductive layers positioned on the upper or lower insulation substrates via conductive holes called through-holes, inner holes or blind holes. On the other hand, corresponding to the recent development of high performance and small-sized electronic equipments, a printed wiring board must be densely loaded with electronic elements. For the purpose of increasing the wiring density, the diameters of the via holes must be reduced.
However, when the diameter of the hole is reduced, and a conventional boring method is used, there are problems in that an inner wall of the bored hole is rough, a position of the hole is inaccurate, and inferior insulation may occur due to the entrance of a plating solution, used in the post plating process, through the inner wall of the hole. To solve these problems, in the past, various improvements have been made to the glass cloth and the matrix resin constituting the insulation substrate. For example, the adhessiveness of a glass cloth with a matrix resin can be improved by selecting a surface treatment agent, the distribution of glass fibers can be made uniform by the fiber-opening treatment and the heat durability can be improved by selecting a matrix resin having a high glass transition point (Tg).
The electronic equipment has become even more dense to require a small hole of 0.2 mm φ or less. However, when a drill corresponding to such a small hole is used, it is easily broken and must be frequently replaced, resulting in a lowering of the productivity. In addition, since the thickness of the insulation substrate is reduced to 0.1 mm or less because of the miniaturization of the printed wiring board, it is difficult to control the depth of the hole to an accuracy of 0.1 mm or less if a drill is used. This is particularly true of a blind via-hole provided in such a thin insulation substrate. Recently, a high density laminated printed wiring board, called a build-up wiring board, wherein an insulation layer and a conductor of 0.1 mm thick or less are sequentially piled up, has been developed, and a method for boring holes in this wiring board by a laser beam has been proposed and carried out.
However, since this build-up wiring board generally has no glass cloth, the dimensional stability and the heat durability are largely degraded, and since a process different from the conventional process is necessary for forming a build-up layer, the production cost is high. Thus, the development of glass cloth having a thickness of 0.1 mm or less, enabling a laminated printed wiring board containing the glass cloth to be bored by using a laser beam, is required.
Generally speaking, an insulation substrate for a printed wiring board is a composite material consisting of a matrix resin composed of organic material and a glass cloth composed of inorganic material, wherein the organic and inorganic materials unevenly exist. Accordingly, the respective materials are unevenly processed during the boring with a laser beam to roughen the inner wall of the hole and deteriorate the reliability of the plating conductivity. This is because the organic material portion and the inorganic material portion have different physical properties such as absorptivity of laser beam energy, decomposition temperature or heat-dispersibility.
As a countermeasure thereto, the process conditions are properly selected to have well-bored holes. However, in the insulation substrate in which the organic matrix resin and the inorganic glass cloth are unevenly distributed in the surface direction, an improvement in process uniformity of the bored holes has not yet been achieved.
To improve the surface-directional evenness, the formation of glass cloth having no gaps has been attempted by increasing a weaving density of the cloth. Accordingly, the uniformity of the respective holes is improved but the processibility is lowered due to the increase in an amount of glass, which also results in an increase in cost.
Recently, IC packages have been high-lighted as a novel use of a high density wiring board necessitating micro-holes. Of them, an area-array type IC package such as BGA or CSP is of a substantially square shape and is required to have the same mechanical properties both in horizontal and vertical directions. Particularly, a thermal expansion coefficient of a laminated board having a significant influence on the connection reliability is required to have the same values in the vertical and horizontal directions. Thus, a weaving density of a glass cloth is required to have nearly equal values both in the warp and weft directions, and a yarn configuration, including flatness and widening degree, must be approximately equal in both the directions.